linux-kernel - Re: [PATCH v1] Revert "drm: bridge: ti-sn65dsi83: Add error recovery mechanism"

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Message-Id: <DEJCGODDOTXT.QT2J4E31GUVW@bootlin.com>
Date: Thu, 27 Nov 2025 09:46:07 +0100
From: "Luca Ceresoli" <luca.ceresoli@...tlin.com>
To: "Francesco Dolcini" <francesco@...cini.it>, "Andrzej Hajda"
 <andrzej.hajda@...el.com>, "Neil Armstrong" <neil.armstrong@...aro.org>,
 "Robert Foss" <rfoss@...nel.org>, "Laurent Pinchart"
 <Laurent.pinchart@...asonboard.com>, "Jonas Karlman" <jonas@...boo.se>,
 "Jernej Skrabec" <jernej.skrabec@...il.com>, "Maarten Lankhorst"
 <maarten.lankhorst@...ux.intel.com>, "Maxime Ripard" <mripard@...nel.org>,
 "Thomas Zimmermann" <tzimmermann@...e.de>, "David Airlie"
 <airlied@...il.com>, "Simona Vetter" <simona@...ll.ch>
Cc: "Francesco Dolcini" <francesco.dolcini@...adex.com>,
 <dri-devel@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>,
 João Paulo Gonçalves
 <jpaulo.silvagoncalves@...il.com>, <stable@...r.kernel.org>, "Herve Codina"
 <herve.codina@...tlin.com>, "Tomi Valkeinen"
 <tomi.valkeinen@...asonboard.com>, "Emanuele Ghidoli"
 <emanuele.ghidoli@...adex.com>
Subject: Re: [PATCH v1] Revert "drm: bridge: ti-sn65dsi83: Add error
 recovery mechanism"

Hello Francesco, all,

On Tue Nov 25, 2025 at 11:38 AM CET, Francesco Dolcini wrote:
> From: Francesco Dolcini <francesco.dolcini@...adex.com>
>
> This reverts commit ad5c6ecef27e ("drm: bridge: ti-sn65dsi83: Add error
> recovery mechanism").
>
> The reverted commit introduces a regression on Verdin AM62, and
> potentially on more devices, not being able to generate a clock
> that the TI SN65DSI83 PLL can lock to, with the display periodically
> blinking.
>
> Verdin AM62 SoM has a Toshiba TC358778 DPI to DSI bridge, that can be
> connected to an LVDS display over a TI SN65DSI83 bridge. Before this
> change despite the TI SN65DSI83 reporting with a debug print a PLL
> locking error the display was working fine with no visible glitches.
>
> The reasons for this issue was investigated without getting to a final
> conclusion:
>
>  - the DPI clock was measure and it is stable/accurate
>  - the DSI clock was not possible to measure, but this setup is used
>    with other display/bridges with no known issues
>  - the DSI clock is configured in continuous mode
>  - the actual DSI clock generated from the TC358778 is generate with a
>    PLL from a 25MHz reference clock
>  - it's not clear why some frequencies are working and some are not, for
>    example 50000000, 68750000, 72750000, 75000000 frequencies are fine,
>    while 69750000, 71100000, 72500000 are not
>
> Given that the safest approach is to just revert the commit, till a
> proper solution for error recovery that is not introducing regression
> is figured out.
>
> Reported-by: João Paulo Gonçalves <jpaulo.silvagoncalves@...il.com>
> Closes: https://lore.kernel.org/all/bhkn6hley4xrol5o3ytn343h4unkwsr26p6s6ltcwexnrsjsdx@mgkdf6ztow42/
> Fixes: ad5c6ecef27e ("drm: bridge: ti-sn65dsi83: Add error recovery mechanism")
> Cc: stable@...r.kernel.org
> Signed-off-by: Francesco Dolcini <francesco.dolcini@...adex.com>

Thanks for having sent this revert patch.

However after evaluating the overall situation I decided to send a
different patch to address this issue in the short term. The idea is to
just ignore the PLL_UNLOCK error, keeping the existing
structure. Rationale:

 * this sloves the issue for Toradex, based on João's initial report
 * there is no evidence of any bugs in the recovery mechanism, it's
   just exposing a pre-existing problem that was only producing a
   non-fatal dev_err() before
 * a full revert would remove error checking for all errors, including
   those not creating any issue, thus removing a useful feature
 * a full revert would require rewriting patches such as [0] (not a big
   deal per se, but see next bullet)
 * after patches such as [0] are applied, re-adding the error recovery
   mechanism would require another rework, so more work for authors,
   reviewers, testers and maintainers

[0] https://lore.kernel.org/lkml/20251112-drm-bridge-atomic-vs-remove-v3-2-85db717ce094@bootlin.com/

Luca

--
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com